BuddyPress.org

I will answer my own question. I called tech support at hostmonster.com and the second guy I spoke with was very helpful. He looked at the .htaccess file in the /subdir and saw that SimpleScripts had not properly configured it. He edited line 2 to read:

RewriteEngine On

RewriteBase /~username/subdir/

Substituting the actual username and subdir for the placeholders I wrote above.

HTH